This invention relates to an improved organic scintillator system having minimized reabsorption effects and useful for the detection of high energy particles and electromagnetic radiation.
A number of plastic scintillators have been developed for the detection of high energy particles and photons , J. B. Birks, "The Theory and Practice of Scintillation Counting", (Pergamon, 1964); T. Inagaki, et al, Nucl. Instr. and Meth. 201 (1982) 511. In a typical system used for the detection of neutrons, the neutron beam interacts with the polymer matrix, elevating some of the monomer units to electronically excited states. Poly(vinyltoluene) (PVT) is often used as the matrix material, since it contains a large number of hydrogen atoms, each of which has a good neutron cross-section. Since the phenyl rings of PVT are aromatic, a .pi..fwdarw..pi.* electronic transition is induced. Liquid scintillator matrices have also been used.
Since many aromatic polymers have low quantum yields of fluorescence, I. B. Berlman, Handbook of Fluorescence Spectra of Aromatic Molecules, 2nd ed. (Academic, 1971), a solute such as 2-phenyl-5-(4-biphenylyl)1,3,4-oxadiazole (PBD) is often introduced. The energy of the .pi.* state of PVT is then transferred to the solute, which fluoresces efficiently. Since primary solutes used for this purpose typically fluoresce at wavelengths shorter than 400 nm, secondary solutes (or "wavelength shifters") are sometimes added. Energy transfer from the primary to secondary solute results in a longer wavelength fluorescence and a better match with the wavelength of optimum response for most photomultiplier tubes.
Ternary and higher order solutes can be added to obtain further wavelength shifts. This is often necessary when using detectors which have optimum responses at much longer wavelengths, as with some photodiodes.
Typical systems include that of U.S. Pat. No. 2,710,284 a solid plastic scintillator, e.g., a vinyl aromatic polymer containing p, p'-diphenyl stilbene and p-terphenyl; USP 2,698,906-p-terphenyl in a hydrocarbon solvent; U.S. Pat. No. 4,292,527 - fiber optic radiation detection system including conventional scintillator systems; U.S. Pat. No. 4,256,900 - fluorescent azolyl benzocoumarin dyestuffs as scintillator components; U.S. Pat. No. 4,326,066--fluorescent triazolyl coumarin dyestuffs as scintillator components; and U.S. Pat. No. 2,188,115--inorganic scintillator.
Unfortunately, energy can be transferred not only from one solute to the next, but also between like molecules of the same solute. Each of these self-transfer steps, when due to the reabsorption of photons, can lead to scintillation efficiency losses. Similar losses can occur in other applications. For example, Fayer et al, R. W. Olson, et al, Appl. Optics 20 (1981) 2934, used computer simulations to predict the degree of light loss due to reabsorption effects in luminescent solar concentrators.
In a plastic scintillator, two types of transfer predominate in the movement of energy from one solute to the next. The first, radiative or "trivial" transfer, involves the emission of a photon by the donor solute and subsequent absorption by the acceptor solute. The probability of this type of transfer is increased by increasing the fluorescence quantum yield of the donor, by increasing the concentration of acceptor, by high acceptor molar absorptivity, and by good overlap between the donor emission and acceptor absorption spectra, N.J. Turro, "Modern Molecular Photochemistry", (Benjamin/Cummings, 1978) Since this type of transfer can take place over large distances, a very high acceptor concentration is usually not required. In a large scintillator, a photon emitted in the interior will pass through a large amount of material before exiting the sample. Therefore, a high optical density can be developed, leading to efficient radiative energy transfer, even with fairly low acceptor concentrations.
The other energy transfer mode is non-radiative or Forster transfer, which involves a coulombic interaction without emission of a photon. This type of transfer is governed by the relation J. B. Birks, "Photophysics of Aromatic Molecules", (Wiley-Interscience, 1970). ##EQU1## where .phi..sub.F is the fluorescence yield of the donor in the absence of acceptor, N' is Avogadro's number per millimole, n is the refractive index of the solution, .epsilon..sub.Q (.upsilon.) is the molar absorptivity of the acceptor as a function of wavenumber, f.sub.M (.upsilon.) is the emission spectrum of the donor normalized to unit area, and .kappa. is a factor determined by the mean relative orientations of the transition moment vectors of the donor and acceptor distributions. For random orientations in rigid media, .sup..kappa.2 =0.475, M. Z. Maksimov, et al, Optika Spec. 12 (1962) 606. The critical radius, R.sub.o, is the donor-acceptor separation at which the probability of donor deactivation for Forster transfer equals the combined probabilities of all other decay routes. The rate constant for Forster transfer, k, is very strongly dependent on donor-acceptor separation. EQU k.alpha.(R.sub.o /r).sup.6
where r is the distance of separation. Since R.sub.o values of 5-30.ANG. are typical, fairly high concentrations of donor and/or acceptor (10-100 mM) are often needed to achieve efficient transfer. Ccnsequently, the relative rates of radiative and non-radiative transfer in a scintillator are a function of solute concentration, F. H. Krenz, Trans. Faraday Soc. 51 (1955) 172.
Considering only these three factors, increasing the concentration of each solute would be expected to increase the total transfer efficiency at each step, thereby giving a larger number of detected photons per incoming neutron. This would be true if the energy in the excited state of each solute were passed only to the solute emitting at the next longest wavelength. However, since most fluorophores have some overlap between their absorption and emission spectra, at high concentrations, self-transfer can occur by reabsorption of light emitted from a solute molecu-e. Each time reabsorption occurs, some quanta of energy are lost due to internal conversion and other deactivation processes (unless the fluorescence quantum yield is 1.0). Moreover, reabsorption events cause an increase in the lifetime of emission, correspondingly decreasing the time response of the scintillator. Reabsorption can also occur with the terminal solute, resulting in a loss of some photons before they can escape the scintillator. Most scintillators exhibit a maximum in their light output versus solute concentration curves. This phenomenon is probably due, in the main, to these reabsorption losses. Forster transfer can also take place between like molecules, but since the solute's excited state lifetime is unaffected, no quanta are lost.
Clearly, one way of increasing scintillator efficiency would be to use only solutes with unit fluorescence quantum efficiency. Then high solute concentrations could be used and, although reabsorption would readily occur, no deactivation would result. In practice, such solutes are rare, which makes this scheme difficult to implement.
All of the references cited above are incorporated by references herein.
As can be seen, there remains a need to improve the energy output, and compoundingly the sensitivity of organic scintillators in response to high energy particles and photons, preferably, by reducing reabsorption losses.